This application relates generally to a method for blow molding plastic containers by using unconventionally hot blow air.
In recent years a body of technology has been developed relating to the molecular orientation of plastic materials in bottles, containers, and other blown articles. For example, the following patents, all incorporated by reference, disclose various techniques and apparatus which are adapted to form a molecularly oriented bottle.
First U.S. Pat. No. 3,599,280 to Rosenkranz discloses a method and apparatus for heating a previously formed parison in preparation for a blow molding operation. U.S. Pat. No. 3,767,747 to Uhlig discloses a method and apparatus for an "extrude, blow, and blow" technique, wherein an extruded tubular parison is enclosed within a first blow mold to form a blown pre-form and then the blown pre-form is enclosed within a final blow mold to form the molecularly oriented final bottle. U.S. Pat. No. 3,781,395 also to Uhlig discloses a method and apparatus similar in many respects to that disclosed in U.S. Pat. No. 3,767,747, but further including the concept of sequentially blowing, stretching, and finally blowing the plastic material.
More recently, U.S. Pat. No. 3,873,660 to Reilly has disclosed a method and apparatus for heat treating blown pre-forms to a temperature within the range for substantial molecular orientation during the final blowing operation.
Specific methods and apparatus have also been adapted to form bottles of particular materials. For example, U.S. Pat. No. 3,733,309 to Wyeth discloses a specific molding method and apparatus to form a bottle comprised of polyethylene terephthalate (PET).
All these patents, however, as well as the generally accepted industry-wide methods, utilize blow air that is substantially at room temperature at the time it is injected into the blowable shape.
As will be appreciated by those skilled in the art, molecular orientation during blow molding will not be effectively accomplished unless the plastic material is blown at a temperature conducive to molecular orientation. Certain specific ranges for optimum temperature ranges are set forth in U.S. Pat. No. 3,781,395, incorporated by reference. Additionally, British Pat. No. 921,308, incorporated by reference, sets forth in Tables I and II temperature ranges that are widely accepted, especially in parison reheat operations prior to blowing. Examples of the temperature ranges listed in the British patent include: 70.degree.-145.degree. C. for polyvinyl chloride, depending upon the percentage of plasticizer present; 100.degree.-160.degree. C. for polypropylene, depending upon the density of the material; and 50.degree.-130.degree. C. for polyethylene, depending upon density. Additionally, it has been found the ideal temperature for polyethylene terephthalate varies directly with the molecular weight, but generally falls within the range of 165.degree.-200.degree. F.
In order to promote molecular orientation it is desirable to blow the blowable shape when the material is at its lowest practical temperature. Due to the adiabatic expansion of the blow air, the material may be cooled during blowing to a temperature at which undesirable results occur.
A first undesirable result is caused by the cold blow air striking a localized region of the blowable shape. This produces a cold spot in the blowable shape, causing differential expansion and variances in wall thickness on the blown article.
In regard to a further undesirable result, Applicants have observed that certain thermoplastic materials, most notably PET, develop a white haze in the bottle wall thickness during blowing. After study of this phenomenon, it now appears that the white haze is actually a region of stress whitening, resulting from a variety of factors: first, the expanding, cooling blow air chills the inside surface of the parison; the material then tends to stress whiten as a result of being expanded and stretched too rapidly for the temperature of the chilled inner surface. This stress whitening may take the form of either stress cracking or stress crazing. When a stress crack occurs, a small void of material is developed, whereas with stress crazing there is no void in the stress-developed flaw. Microscopic examination indicates that the stress whitening observed by Applicants is predominantly of the stress crazing type flaw.
It has been found that incidence of stress whitening may be reduced by reducing the rate of expansion of the blowable shape to form the final article. This may be accomplished by either (1) reducing the blow air pressure or (2) throttling blow air into the blowable shape. Both these procedures, however, consume additional time in the overall blow molding cycle.